In a mixture of a metal power and a lubricant, there have generally been used, as a lubricant, a metal soap such as zinc stearate, and an amide-based lubricant such as ethylenebisstearamide, fatty acid amide, etc. However, in a process for producing a metallic sintered body through powder molding, using a mixture of a metal powder and a lubricant, and then sintering the same at not lower than 500 degrees C. to eliminate the lubricant, there have been the following problems:
1 Stains on Sintered Body
When a metal soap is used as a lubricant, there has been a problem that at the time of sintering, a sintered body gets stains due to residual metallic components contained in the lubricant. To prevent such stains from being caused by the residual metallic components, there have been used, as a lubricant, amide-based lubricants including no metallic components. Using such amide-based lubricants, however, does not provide a complete solution to reduce stains to zero.
2 Surface Defect of Sintered Body
In the case that conventional lubricants are used, they are fused due to friction heat generated on a mold surface at the time of molding, resulting in lubricant agglomerate or mass being formed on the surface of the sintered body. There has been a problem, however, that areas where the lubricants were agglomerated remain as defective areas after they are decomposed at the time of sintering.
3 Strength of Sintered Body
When using conventional lubricants, there has been a problem that strength decreases due to the above-mentioned surface defect, etc.
4 Density of Sintered Material
When using conventional lubricants, a compacting pressure has to be increased to enhance the density of a compact, leading to a problem that a mold is subjected to such a heavy load that it is easily broken. For this reason, it has been impossible to satisfy High Density, High Strength and High Hardness specifications.
5 Decarburization of Sintered Body
When a black lead or graphite is included as an additive, it reacts with air to be decarburized, leading to a problem of decreased strength of a sintered body.